Friday, April 22, 2016

Making a Sharpening Block for a Scrub Plane Iron

For a long time I've wanted to find a better way to sharpen my scrub plane blade.  I've been doing it by hand on the diamond stones, but I tend to change the radius to a MUCH tighter curve than intended.  My scrub plane is a Stanley #5.  I had filed an 8" radius on the business end of the blade and moved the frog back to open the mouth.  My thought was to create a block with a concave surface that I could use with sandpaper to sharpen the blade.
Rendition of the sharpening block with concave top surface
There is an error in my thinking here, but I'll get to that later.  I used my homemade large compass to draw an 8" radius on some cardboard from a cereal box and cut out the shape.
Homemade compass
Then I transferred the convex shape to the end of a squared-up block.  I'm making a convex block first so I can use it to more accurately form the concave block that I'll use for sharpening.  It's much easier to plane a block to a convex shape than to plane a concave shape.
Radius pencilled on block (view from above the block being held vertically in vise)
I planed carefully with the #4 and checked the progress with the cardboard template.
Not close enough
When it was close I used a thin scraper, bending it to task to smooth the shape.
A much better fit
Satisfied, I squared up a piece of wood for the concave block and pencilled in the radius I wanted to plane to.  I used my little wooden compass plane (correct name?) and checked the progress as I went.
Compass plane and scraper
Checking progress - pencil lines show where to remove more
When it was close enough, I bent the scraper to task and smoothed the surface.  But I was going to get the final shape using the convex block I made earlier.  Wrapping it in coarse sandpaper, I went at the concave block until I was satisfied with the shape.
Convex block wrapped in sandpaper
To sharpen the cambered iron, I put the concave block in the vise with some 180 grit sandpaper on it.
Ready to be used
The sandpaper wears out quickly, but it got the job done eventually.  I used the diamond plates to remove the burr and flatten the back, then put some leather with honing compound on the concave block to strop the blade.
Stropping the blade
This worked out quite well.  I did a test cut on some rough sawn stock and it seemed like it was much easier to scrub it than previously.  That was the success.  But here is the failure:
The blade had a tighter radius than the template
I didn't think hard enough about the radius that needed to be transferred to the sharpening blocks.  Look at this schematic I drew up in Sketchup.  Perfect match between the blade and the block of wood.
Blade with 8" radius standing vertically on block with matching curve in top surface
Close-up of above picture
Watch what happens when I angle the blade down so that it is 30° from horizontal.  In this picture, I've moved the blade to the end of the block.
See the gap?
And here is a view from the near end of the block looking straight down to the blade edge.
This view of the edge of the angled blade is the actual radius I should have put on the block
So the question is this: how do I calculate what radius to put on the block?  I have a degree in mathematics and I used to teach geometry (albeit 33 years ago), so I should be able to figure this out!

After trying different things in Sketchup, if I use a 16" radius on the end of the block, that would give just about the right curvature to match an 8" radius on the blade when it is angled at 30°.  Hate to use a brute force method here.  Sure would love a nice geometric solution.


  1. Interesting post. This is something I want to do also as I hate the figure 8 pattern of hand sharpening. Why not use the plane to 'plane' the hollow needed? I would do it in a jig like your 'ready to use pic'. I did good in geometry but not good enough to figure out this angle.

    1. If I had the proper radius on the scrub iron, then that would have been the way to go to make the hollow. But my blade had too tight a radius. But still, that would have been a good way to start making the hollow - not sure why I didn't think of that - maybe the iron was too dull.

  2. And as you discovered the hard way, the profile varied with small changes in angles.
    That's why traditionally, carvers used the actual tool (gouges etc) to cut a piece of wood with the required profile as a honing or stropping block.
    If you do if free hand, the trick would be to replicate that angle for a best fit. If you use a jig, record the required setting. Or simply with the iron still on the plane, gently rub it backward.
    Easy, don't want to create a back bevel, just kiss the blade to keep the edge fresh by removing the inevitable small fractures at the edge. Not sure if I wrote that clearly but you get the gist?

    Bob, the old carver

    1. Thanks for that, Bob. I guess I have a "chicken or the egg" situation. If I had a blade with a true 8" radius, then I could use that to create the sharpening block. And if I had a block with the right concave curve, I could use that to shape the iron. I don't have any kind of grinder, so I have to shape the iron very roughly with mill files and sandpaper. So if I can make a block at least close to the right shape, that will help me sharpen the iron to the proper curvature.

  3. Hi Matt,
    I like your little compass plane. Is it the Paul Sellers model? Have you done it by yourself?

    1. Yes Stefan, that is the Sellers compass plane. I made it last year (from maple) before making the seat for the shop stool. It works pretty well. The tricky part was getting things lined up properly during glue-up.

    2. Hm, I'm fearing a bit more the blade hardening part.
      I will need one for one of the next projects. Actually I'm thinking about reshaping a block plane blade. Don't know if that will work.
      Have you made the iron by yourself?

    3. Hi Stefan. I think a block plane iron would be perfect for this. Just have to plan for it in the width of the plane. For my plane, I bought a length of 1/8" thick, 1 1/2" wide O-1 steel and shaped it with hacksaw and files. That was last year and might have been my first try at hardening and tempering metal. I was very nervous about it - I had never used a torch before. But I used a propane torch, some concrete cinder blocks for safety (to contain the flame - probably didn't need this), and some vegetable oil for quenching. I tempered the iron in the oven for an hour at 350°F. I had read a lot about it before starting. It was fairly easy, but I have no way of knowing whether the blade was properly hardened and tempered. But it seems to be working OK. I think with more experience I'll be able to feel whether an iron is hardened and tempered properly.

  4. If your jig honing surface is cylindrical, your blade edge should be elliptical. As the section of a cylinder by a (geometric) plane is an ellipse.
    Long axis: 2a, small axis: 2b.

    The crown of the blade would be at the end of the long radius.
    Wiki gives the curvature k. (please look because I can't tape it here because of the exponents)
    For x=a and y=0 it gives
    k=a/b² the local radius being 1/k or b²/a=8" in your case

    Although the curvature of the ellipse is not constant, let's suppose taking into account the width of the blade and the crown radius that the variation is negligible across the blade width.
    For a geometric plane cutting a cylinder at 30°, the ratio between a and b is
    b=a sin30°. b being the radius of the cylinder.

    8= b²/a= a²sin²30/a= a sin²30 or
    a=8/sin²30 or
    b=8 sin30/sin²30= 8"/sin 30° =16"
    as you found.


    1. You are absolutely right, Sylvain. I hadn't thought about the elliptical shape - so I guess I sharpened my iron not round, but elliptical. Like you said, in the local 1 1/2" of my iron, the curvature of the ellipse is probably not too different from that of a circle. Great stuff!